1. Field of the Invention
2. Discussion of the Related Art
The invention relates to a degaussing unit comprising a housing which accommodates a disc-shaped thermistor having a positive temperature coefficient of resistance, than thermistor is provided with an electrode layer on two main surfaces and is clamped between two contact springs via the electrode layers. A degaussing unit of this type is commonly referred to as "mono-PTC".
The invention also relates to a degaussing unit comprising a housing which accommodates two disc-shaped thermistors having a positive temperature coefficient of resistance, which are in thermal contact with each other, with the first thermistor having a relatively low resistance and the second thermistor having a relatively high resistance, and the main surfaces of both thermistors being provided with an electrode layer and both thermistors being clamped between two contact springs via said electrode layers. A degaussing unit of this type is commonly referred to as "duo-PTC".
The invention further relates to a cathode ray tube comprising a degaussing coil and a mono-PTC or duo-PTC degaussing unit.
Degaussing units are used, inter alia, in cathode ray tubes, such as color television receivers and color monitors. They serve to demagnetize the shadow mask of the cathode ray tubes the instant said cathode ray tubes are switched on. In this process, an alternating current is sent through a degaussing coil which is connected in series with a thermistor. As said thermistor has a positive coefficient of resistance and is heated by the alternating current, the intensity of the alternating current decreases rapidly. Such a degaussing treatment of the shadow mask leads to a reduction of color deviations in television or monitor images. If necessary, the degaussing unit comprises a second thermistor having a positive temperature coefficient of resistance and a relatively high resistance. Said thermistor is arranged parallel to the first thermistor and to the coil and serves as a heating element for said first thermistor.
Demagnetizing units of the mono-PTC and duo-PTC types are known per se. For example, U.S. Pat. No. 4,357,590 discloses a duo-PTC comprising a high-ohmic thermistor which is arranged in parallel and a low-ohmic transistor which is arranged in series. The main surfaces of the ceramic thermistors are provided with electrode layers which are applied by means of vapor deposition. Said electrode layers are composed of a first layer of a nickel-chromium alloy, a second layer of silver and a third layer of a silver alloy. Since masks must be used to apply the electrode layers by vapor deposition, the extreme edge of the main surfaces of the thermistors is uncovered. The two thermistors are accommodated in a housing (not shown) where they are clamped between two steel contact springs.
The known degaussing unit has drawbacks. For example, it has been found that it cannot withstand the high in-rush currents prescribed in present and future specifications. More in particular, current intensities of 9 A can cause mechanical damage to the known degaussing unit. Visual inspection has revealed that the use of such high current intensities causes pieces of ceramic material to be chipped from the edge of the thermistors and that sparks can be formed at said edge. For these reasons, said known degaussing unit does not comply with the specifications. It has further been found that the same problem also occurs in mono-PTCs to which a vapor-deposited electrode layer is applied in the same manner.